Ion exchange kinetics in removal of small ions. Effect of salt concentration on inter- and intraparticle diffusion

Abstract

Kinetics of ion removal from wastewaters by ion exchange on a commercial synthetic zeolite has been investigated. Dynamic breakthrough profiles have been registered and exploited to determine ion exchange equilibria and mass transport resistances. A simplified dynamic model based on the equation of solid-film driving force has been employed to determine coefficients of mass transport kinetics in the presence of different solutions of regenerating salt. Two diffusional resistances have been found to govern the mass transport kinetics: interparticle diffusion (external film diffusion) and the solid phase diffusion within micropores of the zeolite crystals. At low solution concentration interparticle mass transfer was dominant in the mass transport mechanism, whereas solid phase diffusion dominated at higher concentration. The influence of the salt concentration on mass transfer kinetics has been analyzed using the Nernst–Planck and the Maxwell–Stefan models. The kinetic and isotherm coefficients determined were used to simulate the cyclic process of ion removal: ion exchange and the column regeneration. To verify analysis a generalized dynamic model has been used accounting for directly all the diffusion resistances.